The 2-bit RLE encoding, in addition to supporting colour is also fully
ROMable meaning we can save 32 bytes of RAM per image by switching to
2-bit encoding.

Switch everything in icons and font.clock over to 2-bit encoding.

Note: this requires all the clock PNG files to be reencoded (because
      they were originally in 1-bit grayscale format and this is no
      longer supported by the encoder).

This reduces RAM overhead by 480 bytes and has only a negligable effect
on FLASH usage (+4 bytes).

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

Signed-off-by: Aidan Houlihan <aidandhoulihan@gmail.com>
[daniel@redfelineninja.org.uk: Minor changes to .gitignore]
Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>